327 - 11th Botany Textbook Volume 1

chromatin network and a conspicuous
nucleoli. The chromatin network is the
uncoiled, indistinct and remain thread like
during the interphase. It has little amount of
RNA and DNA bound to histone proteins in
eukaryotic cells (Figure 6.25).
Chromatin is a
viscous gelatinous
substance that contains
DNA, histone &
non–histone proteins and RNA. H1,
H2A, H2B, H3 and H4 are the different
histones found in chromatin. It is formed
by a series of repeated units called
nucleosomes. Each nucleosome has a
core of eight histone subunits.
During cell division chromatin is
condensed into an organized form called
chromosome. The portion of Eukaryotic
chromosome which is transcribed into
mRNA contains active genes that are not
tightly condensed during interphase is
called Euchromatin. The portion of a
Eukaryotic chromosome that is not
transcribed into mRNA which remains
condensed during interphase and stains
intensely is called Heterochromatin.
I Nucleolus is a small, dense, spherical
structure either present singly or in
multiples inside nucleus and it’s not
membrane bound. Nucleoli possesses genes
for rRNA and tRNA.
Functions of the nucleus
• Controlling all the cellular activities
• Storing the genetic or hereditary
information.
• Coding the information in the DNA for
the production of enzymes and proteins.
• DNA duplication and transcription takes
place in the nucleus.
• In nucleolus ribosomal biogenesis takes
place.
6.7.1 Chromosomes
Strasburger (1875) first reported its
present in eukaryotic cell and the term
‘chromosome’ was introduced by Waldeyer
in 1888. Bridges (1916) first proved that
chromosomes are the physical carriers of
genes. It is made up of DNA and associated
proteins.
Structure of chromosome
The chromosomes are composed of thread
like strands called chromatin which is
made up of DNA, protein and RNA. Each
chromosome consists of two symmetrical
structures called chromatids. During cell
division the chromatids forms well organized
chromosomes with definite size and shape.
They are identical and are called sister
chromatids. A typical chromosome has
narrow zones called constrictions. There are
two types of constrictions namely primary
constriction and secondary constriction.
The primary constriction is made up of
centromere and kinetochore. Both the
chromatids are united at centromere,
whose number varies. The monocentric
chromosome has one centromere and
the polycentric chromosome has many
centromeres. The centromere contains a
complex system of protein fibres called
kinetochore. Kinetochore is the region
of chromosome which is attached to the
spindle fibre during mitosis.
Besides primary there are secondary
constrictions, represented with few
occurrence. Nucleoli develop from these
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